This invention relates to the fabrication of high efficiency solar cells, and, more particularly, to the reduction of fabrication cost and manufacturing tolerances for such solar cells and devices.
This invention was made with Government support awarded by Sandia National Laboratory under contract 02-7063A. The Government has certain rights in this invention.
Solar energy offers the potential for providing virtually unlimited energy for use by man, if the solar energy can be made available in a useful form. Much attention has been directed to the problem of using the sun's energy to obtain electricity, which can then be utilized through existing networks. One of the several approaches to generating electricity from solar radiation is the direct generation of electricity by photovoltaic converters.
At the heart of a photovoltaic converter is a photovoltaic cell, also sometimes termed a solar cell. The photovoltaic cell includes a semiconductor material having at least one P/N junction therein. Light falling upon the junction produces mobile electrical carriers in the form of electron-hole pairs, which can then be directed to an external electrical circuit. The operation of photovoltaic cells is described more fully in U.S. Pat. No. 4,234,352, whose disclosure is herein incorporated by reference.
The economics of using photovoltaic cells for many common applications is determined by the efficiency of the cell in converting sunlight to electrical energy. If the efficiency is low, large numbers of cells are required to provide the required electrical current, and the cost of the large number of cells may dictate the use of another energy source. By way of example, the presently most attractive material for construction of photovoltaic cells is silicon. The theoretical maximum conversion efficiency for silicon cells is about 36 percent using sunlight. Cells have been constructed having efficiencies of about 28-29 percent in concentrated sunlight, and about 22 percent in unconcentrated sunlight. However, reaching this level of efficiency has required increasingly complex and costly designs for the photovoltaic cells themselves.
At the present time, the most efficient silicon solar cells are produced with all electrical connections on the back side of the cell, so that no portion of the incident sunlight is shaded by connections and so that efficiency-enhancing treatments such as texturizing can be easily applied to the front side of the cell. The fabrication procedures required to produce these cells utilize multiple, usually at least four, masks and a complex alignment approach. The techniques are similar to those used in the microelectronics industry, but can be even more difficult to apply successfully because of the large surface area of the solar cells that must be processed. As a result, the manufacturing yields of operable cells having high efficiencies is typically low, raising the effective cost of the cells. This increased cost of the cells is one significant consideration inhibiting more widespread use of solar cells in power generation.
There is therefore a need for an improved approach to preparation of solar cells, and the cells made using the improved approach. The design of the cells must not be changed so greatly that efficiency is compromised, in order to achieve the reduced fabrication cost. The present invention fulfills this need, and further provides related advantages.